tracing: tprobe-events: Support multiple tprobes on the same tracepoint

Allow user to set multiple tracepoint-probe events on the same
tracepoint. After the last tprobe-event is removed, the tracepoint
callback is unregistered.

Link: https://lore.kernel.org/all/174343536245.843280.6548776576601537671.stgit@devnote2/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>